AJAX Controls and Asynchronous Postbacks
When working on the client side with AJAX controls and trying to set property values client-side, you may run into issues with asynchronous postbacks with the update panel. The reason is that this approach invokes the event lifecycle of the client-side of the page (init, load, etc.), which your scripts may reside in. If you want certain actions to avoid asychronous postbacks, then you can make use of the page request manager.
This class resides in the Sys.WebForms namespace, and makes use of the getInstance() static method to return an instance to the caller. This class manages the partial lifecycle of the page, and the get_isInAsyncPostBack property to determine whether the current postback is asynchronous. You can use this to prevent your code from doing anything for partial postbacks.